The present invention relates to a rubber composition or, more particularly, to a rubber composition based on natural rubber or a diene-based synthetic rubber having a relatively low Mooney viscosity before vulcanization to give an advantage of good processibility but capable of giving a vulcanizate having excellent mechanical properties such as tensile strength, tear strength, elastic modulus, stress-strain characteristics, elastic resilience and anti-wearing resistance even without formulating a carbon black filler.
In comparison with a rubber composition formulated with a carbon black filler, as is well known, rubber compositions based on natural rubber or a diene-based synthetic rubber and formulated with a non-carbon inorganic filler are inferior in respect of the poor processibility of the unvulcanized rubber composition and the low mechanical properties of the vulcanizate thereof such as tensile strength, tear strength, elastic modulus, stress-strain characteristics, elastic resilience and anti-wearing resistance because the inorganic filler inherently has no affinity with the polymeric rubbery constituent and exhibits no reinforcing effect as a result of formation of the so-called carbon gel formed in the rubber compositions loaded with a carbon black filler.
Various proposals and attempts have been made hitherto to achieve an improvement of rubber compositions in this regard. For example, Japanese Patent Publication No. 51-28623 and Japanese Patent Kokai No. 52-83527 propose a method of adding a sulfur-containing organosilicon compound to such a rubber composition. This method is indeed effective to give substantial improvements in the tensile strength, tear strength, elastic modulus and stress-strain characteristics in the vulcanizates in comparison with those obtained by formulating the composition with carbon black. The degree of improvement achieved by this method, however, is still insufficient requiring, in particular, further improvements in the anti-wearing resistance and mechanical strengths of the vulcanizates at elevated temperatures.